Based upon a recent email to the X.Org developers' mailing list, Canonical is nearing the point of one of their goals for Ubuntu 10.10 of a rootless X Server, or being able to run the X.Org Server without root privileges.

I don't know what you mean by problems with contents of the windows, everything seems to work fine for me.

I mean artifacts that are very short-lived but noticeable, widgets that change their state not all at once, but in bits and pieces... like the outline of a drop-down box remaining shortly after the box itself disappears... window contents when resizing not being refreshed in real-time, but instead getting big black areas prior to the full refresh... just generally anything that requires large-ish refreshes seems to happen in jerks or discrete steps rather than smoothly. Sure it may seem "minor", but compared to Windows and Mac it is noticeably less polished. It's just like Linux font rendering used to be: "fine" but noticeably inferior.... I believe this is what ggeldenhuys is referring to below as X's "poor performance" in comparison to the other platforms.

This should not happen when compositing is on, as the lower layer does not need to be drawed again. See 2D without compositing as one single image. If you alter (with a combobox) one part, then what was under it does not exist anymore, so it need to be recreated after the alteration (combobox) is gone.

With compositing, it is a stack. The image you see is composed of multiple layers, each being on top, or not of an other. They are separated and does not need to each other. If you remove a layer (your combobox), then it will just show a lower layer, no need to as the widget to replain itself (that is slow).

This has nothing to do with X and everything to do with either bad drivers or badly written apps. If you are getting actual glitches (and not just "not quite fast enough" redraws of complicated windows) then it's definitely not an X problem. Windows apps have this too, although since Windows now has compositing, it's not present any more unless you switch to classic mode.